Wavelength division multiplexing communication using an optical fiber is an important technology for transmitting a large amount of information at a high speed. In recent years, a vigorous attempt has been made to provide a smaller-size and lower-cost optical transmitter/receiver module for use in wavelength division multiplexing communication by using a PLC to compose the module.
FIG. 18 is a conceptual view of a conventionally known optical module of PLC type, which is a cross-sectional view taken in a direction parallel to the surface of an optical waveguide substrate. Such an optical module is disclosed in “European Conference on Optical Communication p.629 (1998)” (Document 1). In the present module, a light signal with a wavelength λ1 transmitted through a single-mode fiber 51 is incident on a single-mode optical waveguide 2a formed in a clad 1. The light with the wavelength λ1 is transmitted by a wavelength selection filter 4 disposed in a dicing trench 3, passes through a single-mode wavelength 2b, and is received by a photodiode 52 mounted on a device mounting portion 5a formed in the same single substrate as the optical waveguides. From a semiconductor laser 53 mounted on a device mounting portion 5b formed in the same substrate as the optical waveguides, a light signal with a wavelength λ2 is incident on a single-mode optical waveguide 2c. The light with the wavelength λ2 is reflected by the wavelength selection filter 4, passes through the single-mode optical waveguide 2a, and is incident on the single-mode fiber 51 to be transmitted to the outside. Thus, the present module structure allows transmission/reception by wavelength division multiplexing using the light beams with the wavelengths λ1 and λ2. A photodiode 54 is for monitoring a light output from the semiconductor laser 53.
FIG. 19 shows a conceptual view of another conventionally known optical module of PLC type, which is a cross-sectional view taken in a direction parallel to the surface of an optical waveguide substrate. Such an optical module is disclosed in “European Conference on Optical Communication p.312 (1999)” (Document 2). In the present module, a light signal with a wavelength λ1 transmitted through a single-mode fiber 51 is incident on a single-mode optical waveguide 2a formed in a clad 1. The light with the λ1 is transmitted by a wavelength selection filter 4 fixed by adhesion to an end of the substrate and received by a photodiode 52. From a semiconductor laser 53, a light signal with a wavelength λ2 is transmitted in the same manner as in the module shown in FIG. 16, which will be described later. Thus, the present structure also allows optical transmission/reception by wavelength division multiplexing.
FIG. 20 shows a conceptual view of still another conventionally known optical module of PLC type, which is a cross-sectional view taken in a direction perpendicular to the surface of an optical waveguide substrate. Such an optical module is disclosed in the Document 2 or in Japanese Patent Laid-Open No. 2000-249874. In the present module, a light signal with a wavelength λ1 transmitted through a single-mode fiber 51 is incident on a single-mode optical waveguide 2d formed on a substrate 41. The single-mode optical waveguide 2d is formed between a lower clad la and an upper clad 1b. In the optical waveguide, a wavelength selection filter 4 is inserted obliquely to the surface of the substrate 41. In the present module, the wavelength selection filter 4 is imparted with a wavelength characteristic opposite to that of the wavelength selection filter 4 used in each of the foregoing two conventional embodiments. Specifically, the present module uses the wavelength selection filter 4 which reflects light with a wavelength λ1 for reception and transmits a wavelength λ2 for transmission. Accordingly, the light with the wavelength λ1 incident on the single-mode optical waveguide 2d is reflected by the wavelength selection filter 4 and received by a photodiode 52 disposed on the surface of the optical waveguide. From a semiconductor laser 53, a light signal with the wavelength λ2 is incident on the single-mode optical waveguide 2d. The light with the wavelength λ2 is transmitted by the wavelength selection filter 4, passes through the single-mode optical waveguide 2d, and is incident on the single-mode fiber 51 to be transmitted to the outside. Thus, the present structure also allows transmission/reception by wavelength division multiplexing.
Japanese Patent Laid-Open No. 2002-6155 discloses an optical multiplexer/demultiplexer using an optical waveguide with an optical filter, which is constituted such that an optical multiplexing/demultiplexing portion is composed of a multi-mode interference optical waveguide for increased tolerance on the displacement of the optical filter and respective optical waveguides on the incidence side and on the reflection side have a specified distance therebetween at a coupling point with the multi-mode interference optical waveguide.
Thus, in any of the structures, the use of the foregoing conventional modules allows optical transmission/reception by wavelength division multiplexing. However, the following problems are likely to occur when the foregoing conventional embodiments are put into practical use.
In the conventional embodiment shown in FIG. 18, e.g., the photodiode 52 and the semiconductor laser 53 are mounted on the same single substrate so that electrical and optical insulation therebetween is weak and cross talk presents a problem. Specifically, an electric signal for modulating the semiconductor laser 53 readily affects the photodiode 52 via the substrate, while light leaked from a light beam from the semiconductor laser 53 upon incidence on the single-mode optical waveguide 2c passes under the wavelength selection filter 4 and also readily affects the photodiode 52 via the substrate. The present structure also has the problem that the displacement of the wavelength selection filter 4 causes an excess insertion loss in the path of reflected light. This is because axial displacement occurs between the light reflected from the filter and the optical waveguide on which the reflected light is incident. In other words, axial displacement occurs between the single-mode optical waveguide 2c and the single-mode optical waveguide 2a. Since the position of the wavelength selection filter 4 is determined by the dicing trench 3, the excess loss can be suppressed if positioning accuracy indicing is increased. In this case, however, the process steps are complicated to incur higher fabrication cost. As for the light transmitted by the filter, it does not undergo axial displacement in the path even when the filter is displaced so that dicing accuracy does not present a problem.
In the conventional embodiment shown in FIG. 19, the photodiode 52 is disposed on a substrate different from that of the semiconductor laser 53. In addition, the wavelength selection filter 4 covers the entire end of the substrate so that the problem of cross talk is negligible both electrically and optically. However, the structure shown in FIG. 19 is similar to the structure shown in FIG. 18 in that an excess insertion loss is likely to occur in the path of the reflected light due to the displacement of the filter.
In the conventional embodiment shown in FIG. 20, the wavelength selection filter 4 should be inserted obliquely to the surface of the optical waveguide. Consequently, dicing for inserting the wavelength selection filter 4 should also be performed obliquely. However, the step of performing oblique dicing with respect to the surface is more complicated than the step of perpendicular dicing so that mounting cost is likely to be increased. In the present structure, due to the arrangement of element components, stray light from the semiconductor laser 53 that has leaked out of the single-mode optical waveguide 2d readily enters the photodiode 52 so that optical cross talk presents a problem. To prevent the cross talk, it is necessary to dispose a filter for cutting light with the wavelength λ2 for transmission between the wavelength selection filter 4 and the photodiode 52. However, a complicated mounting process is required to place the wavelength selection filter, the filter for cutting the light for transmission, and the photodiode in succession on the surface of the optical waveguide so that a higher cost for the module is incurred. To prevent this, the foregoing example disclosed in the Document 2 uses a device in which a semiconductor layer for absorbing the light with the wavelength λ2 for transmission is provided in the photodiode 52, thereby achieving simpler mounting. However, such a device is special and less available so that it presents an obstacle when the optical transmitter/receiver module is mass produced at low cost as a general-purpose product. Since the absorption wavelength of a semiconductor is dependent on temperature, if the present device is used, the reception characteristic thereof is easily degraded by a temperature change.
In the foregoing conventional embodiment disclosed in Japanese Patent Laid-Open No. 2002-6155, the multi-mode optical waveguide in which the filter is inserted is long so that the structure of the entire module is increased in size. It may also be considered that the reflected light from the filter readily returns to the inside of the single-mode.
Thus, it has been difficult to fabricate an optical module with excellent characteristics at low cost even with the foregoing prior art technologies. The present invention has been achieved to solve the foregoing problems.